All 356s (coupes, cabriolets, speedsters, “D”s, roadsters) from the mid-50s until the advent of the 911 used a variation of the same Porsche air-cooled flat-4 engine (with the exception of the Furmann 4-cam GS/GT). 912s from 1965-1969 also used a variation of this engine.
The early Porsche engine shared the basic architecture (and some bits and pieces) with the VW Type-1, but they are just enough different that most of the internals are not interchangeable. The cases are different, the heads are different, the crankshafts are different, etc. As Al said— the transmissions are pretty easily interchangeable (adapter plates are not required).
VW and Porsche developed an engine jointly in the late 60s for manufacture and use in the Porsche 914 and VW Bus. This engine was called the Type-4. It also shares the basic architecture with the early Porsche 4, and the VW Type-1, but is its own animal. Again, there are a few bits that are interchangeable, but the cases, heads, and internals are all different. Type-4s are bigger and more rugged than either other air-cooled Flat-4. They are also quite expensive. Again, no adapter plate is required for these engines.
Any of these engines can have EFI and electronics spark— but as Phil said, the vast majority of them have carburetors and distributors. It’s worth noting that there is no “off-the-shelf”, readily available electronic package for purchase for a high-performance air-cooled flat-4. Mexican Beetles had EFI and electronic spark, but they were by no means a high-performance engine. Guys who go down this road put together their own packages, getting throttle bodies from here, logic modules from there, etc. It’s super-cool, but a hobby within a hobby, and probably fun for about 5% of prospective market.
On the air-cooled side, the old 356 engines are becoming horrifically expensive, and don’t offer much more than a Type-1– I’d leave these for guys with original cars who put a lot of value having a “correct” engine. The Type-4 is a highly desirable engine, and if you have deep pockets it is clearly superior to either of the other two options, but it’s not hard to spend $20,000 on a properly modified Type-4. In the V8 world, you can get close to 1000 hp for that kind of money.
... which leaves the Type-1 as the air cooled engine that most people choose. They take a lot of maintenance, they are somewhat fragile, but they are erector-set of the automotive world. They can be configured in almost any arrangement from a 60 hp 1600 to a 2332 making over 200 hp. There are lots of aftermarket companies casting many variations of heads, several variations of cases, etc. camshaft choices, lifter choices, really everything – there are several companies making all of it. It’s not like speed parts for American V-8’s, but it’s close.
It’s worth noting that any VW or Porsche air cooled flat-four will bolt up to any VW or Porsche air cooled transaxle, including the 901 & 915 Porsche 5-speeds. A G50 Porsche would also probably bolt up, but it’s enormous and heavy. There will need to be changes to some of the flywheels and some of the clutches, but no adapters will be required.
Subaru is a whole ‘nuther world. There are shops by the bazillion getting freaky amounts of power out of turbo engines. There is significantly less aftermarket support for normally aspirated engines, but a stock EJ25 is robust enough that (even unmodified) it makes as much power as a pretty heavily modified Type-1. Outfront has a high performance NA EJ25. There are one or two guys running a turbo WRX engine in their cars, but the work involved to make this an option is not for the faint of heart. Anything is possible, given enough money.
Nothing bolts up with this engine. The choices are to either run the Subaru transaxle with a reverse ring/pinon, or use an adapter plate with a VW or Porsche transaxle. Of course, putting a water-cooled engine in what was originally an air-cooled vehicle adds a significant amount of complexity. But those who have done it feel that the increase in longevity and ease-of-use is worth the effort.
There also those who believe that going in this direction alters the essence of the car to the extent that it becomes something else altogether... but those people are self-confessed retro-grouches and generally have a co-dependent love/hate relationship with their engines. They often have plenty of time to contemplate the relative merits and shortcomings of their chosen platform, as they sit beside the road waiting for a flatbed. These people are outliers and social misfits, generally derided as gearheads in high school and afterward. They are the rugged individualists who scoff at modernity, with all it’s seamless capability.
That’s the lay of the land, as the hobby presently is configured. These are my opinions, not necessarily the opinions of staff or management. Your mileage may vary.
. I was/am going towards the suby engine because of reliability, I prefer not to be tinkering all the time with the older VW engine, and I have zero experience with either to be honest but now I am going back and forth between the suby and VW engine. 
